Hitherto, to accurately detect a small signal current flowing in a circuit in a communication device, for example, a method of connecting resistors in series in the circuit and measuring a voltage drop in the resistors is used. In this case, however, a load different from that in a communication system is applied, and there is the possibility that an adverse influence is exerted on the communication system. Consequently, a method of indirectly measuring a signal current by detecting the gradient of a current magnetic field generated by the signal current (while maintaining an insulated state so that direct current does not flow) is used. Examples of such a method are a method of using a transformer in which coils are connected to each other and a method using a photocoupler.
In the method of using the transformer, although a signal current which is an alternate current can be transmitted, a signal current which is a direct current cannot be transmitted. Moreover, a frequency band in which the signal current as the direct current can be transmitted is limited. At a frequency other than in the proper frequency band, an impedance of the transformer changes, so that it lacks linearity, and an adverse influence may be exerted on the communication device and the like. On the other hand, the photocoupler has excellent frequency characteristics, but it is difficult to reduce the size when the photocoupler is used. There is also a problem such that, in long terms, deterioration in a signal intensity caused by a change with time tends to occur.
Consequently, a current sensor in which a giant magnetoresistive element (hereinbelow, GMR element) producing a giant magnetoresistive effect is disposed in the current magnetic field generated by the signal current to detect the gradient of the current magnetic field has been proposed (refer to, for example, U.S. Pat. No. 5,621,377). A current sensor using such a GMR element has an excellent detection sensitivity and responsiveness and can obtain detection characteristics which are stable also against a temperature change.
Recently, a current sensor capable of detecting a weaker current and having a more compact general configuration is being demanded. In a current sensor using a conventional GMR element, however, a GMR element is provided so as to be adjacent to a line to be measured in an in-plane direction, so that it is difficult to detect a weak current and it is disadvantageous from the viewpoint of miniaturization.